1. Field of the Invention
The present invention relates generally to optical communication systems. In particular, the invention relates to optical multiplexer/demultiplexer systems that utilize interference filters.
2. Background Technology
It is generally understood that one way to expand the bandwidth of an existing optical fiber is to multiplex many optical signals having slightly different wavelengths into the fiber. Complete wavelength-division multiplexer systems are often composed of simpler multiplexers having two or more optical input signals, each of which may or may not be multiplexed, and a single output optical signal that combines the optical input signals. Similarly, complete demultiplexer systems are often composed of simpler demultiplexers. The simplest multiplexer has two inputs and one output optical signal, while the simplest demultiplexer consists of one input and two output optical signals.
There are performance expectations of wavelength-division multiplexers and demultiplexers that must be considered in their design. First, the devices are expected to work in the presence of frequency drift in the carrier frequencies. For example, the International Telecommunication Union (ITU) has established standards for the frequencies to be used in WDM devices. One standard includes carrier frequencies spaced 100 GHz apart, centered around 194.100 THz. However, the actual frequency of the carrier may drift by as much as 25 GHz. It is therefore required of a WDM device that it work effectively over a passband of 50 to 60 GHz for each channel.
Another requirement is that when demultiplexed, the energy of other channels should be essentially eliminated from a given channel. This is measured by the isolation figure of merit and it typically must be better than −30 dB (0.1%). Because of the nature in which simple demultiplexers are combined to make complete demultiplexers, this figure of merit usually applies to each component as well as to the entire system. The isolation should be achieved across the entire passband of each of the other channels.
Insert loss is the measure of how much energy remains in the output of a device relative to the input. For demultiplexers and multiplexers this should be better than −6 dB (25% retention) and preferably better than −3 dB (50%). Because simple multiplexers or demultiplexers are cascaded together, the insertion loss must be much better for a simple device than for the complete system. The exact value depends on the architecture of the composite multiplexer, which will be discussed below.
Return loss measures the amount of energy that reflects from the device back into the input channel. Because the effects of this reflected energy might disrupt performance in upstream devices, the return loss must be extremely low. Typical values of −40 dB (0.01%) are expected. To guarantee system-level compliance, simple multiplexers and demultiplexers must attain this degree of return loss also.
There are many ways to achieve wavelength-division multiplexing and demultiplexing. Methods include the use of diffraction gratings, Bragg (volume) gratings, and Mach-Zehnder interferometers. Each of these approaches relies on interference physics to separate the very closely spaced frequencies.
Another interference technique involves the use of etalons and multilayer optical interference filters. An etalon consists of two parallel reflecting surfaces and a cavity between them of a very precise length. The etalon transmits an optical frequency only if the cavity length is approximately an integer multiple of one-half of the corresponding wavelength. Other frequencies are reflected from the etalon. It should be noted that an etalon has many transmission peaks and the distance between these peaks when measured in frequency is called the free spectral range, which is inversely proportional to both the cavity length of the etalon and the index of refraction of the material inside the cavity. The passband and isolation of an etalon are a function of the reflectivity of the reflecting surfaces. For applications requiring very narrow passbands and very high isolation, such as spectroscopy, etalons are an ideal device. Unfortunately, simple etalons are not well suited to WDM applications. Etalons with sufficient isolation of the reflected channels do not provide sufficient passbands for the transmitted channels. Conversely, etalons with sufficient passbands do not provide sufficient isolation from the unwanted frequencies.
Use of multilayer interference filters is a way of overcoming this problem. Essentially, multilayer interference filters are cascaded etalon structures. By cascading many etalons, it is possible to achieve both high isolation and sufficient passbands. Multilayer filters, often called stacks, sometimes consist of one hundred or more layers of alternating materials having differing indices of refraction. The interface between any two layers serves as a reflector and the composite effect of many weak etalons is a filter with relatively wide passbands and high isolation. They are therefore good candidates for WDM devices.
However, there are practical considerations of multilayer interference filters that must be considered in the design of WDM devices that incorporate them. First, a single optical filter, by itself, will produce unacceptable isolation or return loss. Consider a typical optical filter that transmits −0.04 dB (99.08%) of the energy in the frequencies intended to be transmitted (hereinafter the even-parity frequencies) and reflects −0.0004 dB (99.9908%) of the energy in the frequencies intended to be reflected (hereinafter the odd-parity frequencies). The insertion loss requirements are clearly met by this filter for both subsets of frequencies. Considering for the moment the demultiplexing case, the isolation of the odd-parity frequencies from the even-parity frequencies is −40 dB, which is quite acceptable. However, the isolation of the even-parity frequencies from the odd-parity frequencies is only −20 dB, which is not acceptable. In the case of multiplexing, these numbers correspond to the return loss and are equally unacceptable. This performance is typical of optical interference filters and must be compensated for in a practical multiplexing or demultiplexing device.
Another practical concern is that the frequency response of an optical interference filter is dependent on the incident angle. For polarization reasons, optical filters are best designed for normal incidence. The free spectral range of the optical filter is inversely proportional to the incident angle. Assuming that the maximal allowable shift of the transmission peak of off-axis components is 10 GHz, the maximum allowable angular deviation is approximately 1 minute for typical WDM frequencies. This imparts a serious constraint on the planarity and parallelism of the layers of an optical interference filter.
The angular constraints imposed by the optical filter also lead to limitations on the degree of miniaturization that can be achieved. Assuming that the incident beam is a collimated Gaussian beam, the divergence angle of the beam is inversely proportional to the beam width. For a divergence angle of 1 minute, the beam diameter must be approximately 5 mm. This assumes that there are no other sources of angular divergence, which in practice may not hold. In practice the beam may be 10 mm or larger to account for other sources of angular deviation.
The chromatic dispersion (the frequency-dependence of the index of refraction) of potential materials to be used in optical interference filters must also be considered. Consider an optical filter designed to have a transmission peak at the central frequency of the ITU frequency grid. This frequency is an integer multiple of the free spectral range of the filter. The free spectral range itself is typically an integer multiple of the ITU grid spacing so that other transmission peaks will align with the grid. However, the free spectral range is inversely related to the index of refraction. It therefore follows that the free spectral range is frequency dependent. If the index of refraction changes at the extremes of the communication band, the free spectral range will no longer be equal to that of the design, resulting in a shift of the transmission peak from its intended position on the ITU grid. The tolerances on dispersion are quite tight. For a 10 GHz allowable shift over a 5000 GHz communication band, centered at 194 THz, the dispersion, as measured by the rate of change of the index of refraction, must be not greater than 4×10−8/GHz. This rules out many materials commonly used in optical interference filter designs.
In theory it is possible to design an optical filter with very steep transitions from reflection to transmission mode. In practice, there are a limited number of layers that can be deposited accurately. As a result, the transition may not be steep enough to provide all performance measures. For example, suppose a demultiplexer must provide a bandwidth of 50 GHz, measured as the −3 dB transmission point, and −30 dB isolation for a communication system with 100 GHz channel spacing. The transition must therefore be from −3 dB to −30 dB in a 50 GHz band. This may prove difficult for some optical filters.
It is also possible in theory to design optical filters with an arbitrary free spectral range, provided that the transmission peaks are integer multiples of the free spectral range. In practice this is difficult for smaller free spectral ranges since the required thickness of each layer is inversely proportional to the free spectral range. Optical filters with a free spectral range of 200 GHz are very difficult to produce since, even for high indices of refraction, the corresponding thickness is several hundreds of micrometers. It is difficult to produce many repetitive layers of material films to such thicknesses while maintaining the necessary planarity and parallelism required. Materials with high indices can reduce the required thickness but are typically too dispersive to be used across an entire frequency band.
It is also possible to fabricate narrow bandpass filters that have only one transmission peak. These are usually constructed with a cascade of etalons with differing free spectral ranges. In the bandpass filters there is only one frequency that can be integrally divided by the various free spectral ranges. Because these filters function over a small frequency band, they are not prone to the dispersion limitations mentioned above and are therefore easier to fabricate.
One conventional way to couple multiplexers and demultiplexers is shown in FIG. 1 where a series of optical filters 1 through N−1 are arranged in a linear stage. When configured as a demultiplexer, the output of each optical filter stage is a single frequency optical signal 1 though N−1. This technique for filtering signals is disadvantageous because some optical signals must traverse a large number of optical filters, compounding the performance requirements of each stage to unreasonable levels.
In another conventional approach, a binary tree structure is used to multiplex or demultiplex optical signals. FIG. 2 shows the transmission curves in an ITU grid for this approach for a series of filters A-O. applied to the binary tree structure. If it is assumed that the optical signals reside on the 100 GHz ITU grid, the first stage has a 200 GHz free spectral range filter. In a demultiplexer mode, this stage separates every other frequency, or alternating frequencies, into the odd-parity or even-parity sets. Each subsequent stage has double the free spectral range.
There are several disadvantages with this approach. First, the requirement of the 200 GHz optical filter over the entire communication range hinders practical implementations. Second, the 200 GHz free spectral range leads to impractical thicknesses for the optical layers. Additionally, the need to function over the entire spectral range requires materials with very little dispersion. Combining these requirements limits the available materials that can be used to produce the optical filter. Considering that the optical filter needs to be on the order of 10 mm in diameter, it would be very difficult to obtain high yields with limited material choices.
Accordingly, there is a need for improved multiplexer/demultiplexer devices that avoid or overcome the foregoing difficulties.